Monolayer MSi2N4 (M = Mo, W) has been fabricated and proposed as a promising channel material for field-effect transistors (FETs) due to the high electron/hole mobility. However, the barrier between the metal electrode and MSi2N4 will affect device performance. Hence, it is desirable to reduce the barrier for achieving high-performance electrical devices. Here, using density functional theory (DFT) calculations, we systematically investigate the electrical properties of the van der Waals (vdW) contacts formed between MSi2N4 and two-dimensional (2D) metals (XY2, X = Nb, Ta, Y = S, Se, Te). It is found that the contact types and Schottky barrier height (SBH) of MSi2N4/XY2 can be effectively tuned by selecting 2D metals with different work functions (WFs). Specifically, n- and p-type Schottky contacts and Ohmic contacts can be achieved in MSi2N4/XY2. Among them, MoSi2N4/H-NbS2, WSi2N4/H-XS2, and WSi2N4/H-NbSe2 present Ohmic contacts due to the high WF of 2D metals. Notably, the pinning factors of MSi2N4/XY2 are obviously larger than those of the other 2D semiconductor/metal contacts, indicating that the Fermi-level pinning (FLP) effect is weak in MSi2N4/XY2. Therefore, vdW stack engineering can strongly weaken the FLP effect, making the Schottky barrier tunable in MSi2N4/XY2 by choosing 2D metals with different WFs. The results provide important insights into the selection of appropriate electrodes and valuable guidance for the development of MSi2N4-based 2D electronic devices with high performance. 

Tunable Schottky Barrier and Efficient Ohmic Contacts in MSi2N4 (M = Mo, W)/2D Metal Contacts

Two-dimensional (2D) materials have enormous applications and are widely studied in the field of electronics and optoelectronic devices. The electrical contacts between the 2D materials and the electrodes seriously affect the performance of the 2D material optoelectronic devices. Exploring the electronic structure and transport behavior at the interface between 2D materials and electrodes and the underlying physical causes will contribute to the development of 2D material integrated circuits and optoelectronic fields. In this study, the charge transport barriers at the Au/MoS2 and graphene/MoS2 interfaces were investigated from both the Schottky and tunneling barriers through theoretical calculations, which demonstrated that the graphene/MoS2 interface has not only a very low Schottky barrier but also a low tunneling barrier compared to the Au/MoS2 interface. Then, three types of optoelectronic devices, Au/MoS2/Au, graphene/MoS2/graphene, and graphene/MoS2/Au field-effect transistors (FETs), were fabricated. Compared with the Au/MoS2/Au device, the graphene/MoS2/graphene device shows an excellent optical response (R = 654 mA W–1 at 532 nm laser with a power density of 4.8 mW cm–2) and ultrafast response time (rise time 9.8 ms, 12.8 ms fall time). The graphene/MoS2/Au device exhibits excellent rectification behavior and optoelectronic response, with almost no current and photocurrent gain and cutoff state under reverse bias, excellent optoelectronic response under positive bias (responsivity R = 293.4 mA W–1 at a power density of 4.8 mW cm–2 for a 532 nm wavelength laser), and excellent response time (rise time 12.55 ms, fall time 15.44 ms). Our studies are expected to bring opportunities for highly sensitive, high-speed, and energy-efficient photodetectors for comprehensive applications. 

Excellent Optoelectronic Properties and Low Contact Resistance of Graphene/MoS2 Heterostructure Optoelectronic Devices: First-Principles Calculation and Experimental Verification

The semiconductor industry has been facing obstacles in designing low-power (LP) nanoelectronic devices due to the lack of accurate theoretical simulations and low-resistance contacts of heterojunctions. Here, based on experimental data, we discover that the generalized gradient approximation using the Fritz Haber Institute pseudopotential accurately describes the lattice constants and electronic properties of monolayer ${\mathrm{Mo}\mathrm{Si}}_{2}{\mathrm{N}}_{4}$. It is further confirmed that this method is also applicable to characterize the electrical contact properties and device transport characteristics of metal$/{\mathrm{Mo}\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ heterojunctions (where metal includes $\mathrm{Sc},\phantom{\rule{0.2em}{0ex}}\mathrm{Bi},\phantom{\rule{0.2em}{0ex}}\mathrm{Ag},\phantom{\rule{0.2em}{0ex}}\mathrm{Al},\phantom{\rule{0.2em}{0ex}}\mathrm{Ti}$, graphene, $\mathrm{Cr}$, $\mathrm{Fe},\phantom{\rule{0.2em}{0ex}}\mathrm{Cu},\phantom{\rule{0.2em}{0ex}}\mathrm{Co},\phantom{\rule{0.2em}{0ex}}\mathrm{Au},\phantom{\rule{0.2em}{0ex}}\mathrm{Pd},\phantom{\rule{0.2em}{0ex}}\mathrm{Ni},$ and $\mathrm{Pt}$). Calculations show that various bulk metal/${\mathrm{Mo}\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ heterojunctions exhibit n-type Schottky barrier contact features, and it is possible to achieve 100% carrier injection for ${\mathrm{Mo}\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ with $\mathrm{Sc},\phantom{\rule{0.2em}{0ex}}\mathrm{Ag},\mathrm{Ti},\mathrm{Pd},$ and $\mathrm{Pt}$ contacts. Using the Schottky-Mott rule and quantum transport simulation, a strong Fermi-level pinning effect at the metal$/{\mathrm{Mo}\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ interfaces is observed, and $\mathrm{Sc}$ is identified as the best metal electrode. The 5.1-nm p-i-n field-effect transistor (FET) with $\mathrm{Sc}$ electrodes can meet the off-state current requirement of the 2013 International Technology Roadmap for Semiconductors standard for high-performance and LP FET devices. By employing a high-k gate, the on-state current for the undoped LP device can be improved by 2 orders of magnitude, and its high on:off ratio is up to 3 \ifmmode\times\else\texttimes\fi{} ${10}^{6}$. These findings provide insights into the metal/${\mathrm{Mo}\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ interface behaviors and provide a theoretical reference for designing ${\mathrm{Mo}\mathrm{Si}}_{2}{\mathrm{N}}_{4}$-based nanoelectronic devices by selecting suitable electrodes with low contact resistance. 

First-principles study of metal-semiconductor contacts and quantum transport simulations for 5.1-nm monolayer <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:msub><mml:mrow><mml:mi>Mo</mml:mi><mml:mi>Si</mml:mi></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mrow><mml:mi mathvariant="normal">N</mml:mi></mml:mrow></mml:mrow><mml:mn>4</mml:mn></mml:msub></mml:math> devices

Contriving an industry-compatible method for fabricating purely high-quality p-type two-dimensional (2D) field effect transistors (FETs) has been proved to be pressing but challenging due to the strong interfacial coupling and thus tight Fermi level pinning (FLP). Herein, by employing density functional theory calculations, a straightforward but effective strategy is proposed to achieve high-quality p-type contact in few-layer TeO2–Pt junction by using a build-in intercalation to suppress the FLP and shield the damage from electrodes to the excellent electrical properties of TeO2 such as its ultra-high hole mobility. Specifically, the increase of TeO2 layer number will make the charge distribution of VBM and CBM be more lumped in the inner core layer, as a result, the outlying Te–O sublayer can act as an inserting layer like BN to protect the semiconducting electronic states and avoid gap-state pinning. Thus, as the TeO2 layer increases from 1 L to 3 L, (Ⅰ) strong FLP in metal-TeO2 interfaces is gradually unpinned; (Ⅱ) the polarity of Pt–TeO2 contact reverses from n-type to p-type; (Ⅲ) the excellent electrical properties of TeO2 such as hole effective mass is preserved as original despite strong charge transfer occurring at the interface. Besides, the tunnelling barrier at the interface is close to zero. This is quite different from introducing an extern BN intercalation, in which case the tunnelling through BN will prominently reduce the overall hole current. In addition, the contacts between 2D metal electrodes and monolayer TeO2 are found to obey the Schottky-Mott rule, and thereby p-type Ohmic contacts can be obtained at TeO2-VS2, -NbS2 interfaces. Whereas, a relatively large tunnelling barrier exists at the 2D metal-TeO2 interfaces. Our findings are quite essential and beneficial for designing high-performance p-type 2D FETs. 

Realization of Fermi level unpinning and high-quality p-type contacts for 2D β-TeO2 by a built-in intercalation

Schottky-Ohmic transition and tunable ferromagnetism in van der Waals monolayer <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Cr</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>Ge</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>Te</mml:mi><mml:mn>6</mml:mn></mml:msub></mml:mrow></mml:math> heterostructure

Strong Fermi-level pinning (FLP) always occurs at the two-dimensional (2D) semiconductor-metal interface due to the complex interfacial charge transfer. By using monolayer (ML) Bi2OS2, an emerging 2D semiconductor with the highest electron mobility, the Schottky barrier heights (SBHs) and origin of charge transfer at Bi2OS2-metal interfaces are systematically studied based on density functional theory calculations. In 3D metal-Bi2OS2 interfaces, the formation of chemical bonding and the effect of Pauli exchange repulsion are found to be responsible for the strong interfacial charge transfer, resulting in strong FLP, and the direction of charge transfer induced by the two factors is opposite. Besides, an extra interfacial charge transfer is expected to equilibrate the Fermi level when the work functions (WFs) of metal electrodes are out of the range of electron affinity energy and ionization energy of semiconductors. For 2D metal-ML Bi2OS2 interfaces, surprisingly, the FLP is found to be entirely suppressed, and thereby, the 2D metal-Bi2OS2 contacts obey the conventional Schottky-Mott model. Such intriguing behavior arises from the 2D metal electrodes chosen in this work can effectively shield the effect of Pauli exchange repulsion. Consequently, wide-range and linear-tunable SBHs can be obtained and the conversion from n-type Ohmic contact to p-type Ohmic contact can be achieved by using 2D metal electrodes with different WFs. This study not only provides a theoretical foundation for selecting favorable metal electrodes in devices based on ML Bi2OS2, but also helps to enhance the understanding of the mechanism of interface interaction between metals and 2D semiconductors. 

/pdf/schottky-barrier-heights-and-mechanism-of-charge-transfer-at-26cc5z4m.pdf

Schottky barrier heights and mechanism of charge transfer at metal-Bi2OS2 interfaces

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Schottky barrier heights and mechanism of charge transfer at metal-Bi2OS2 interfaces